1. Field of the Invention
The present invention relates to a polyester raw material, and more particularly to a polyethylene terephthalate raw material (PET raw material), which contains finely divided, disperse, inorganic and/or organic particles. The invention further relates to films produced therefrom which have improved winding properties and are therefore more suitable than conventional films for use as capacitor films.
2. Description of Related Art
Thinner and smoother films are in demand as dielectrics, particularly for use in capacitors, in order to reduce the physical size of capacitors or to increase the capacitance. However, the processing of ever thinner, conventionally stretched films increasingly causes the problem that irreversible film defects, such as the formation of folds and stretches, occur in the various processing steps, such as winding, metallization, cutting and capacitor winding. In order for the films to be processed without folding and stretching, they require adequate slip, which prevents the films from blocking in the individual process steps, and also a surface topography which enables the air between the individual film layers in the winding to escape sufficiently quickly. Attempted film processing solutions, such as increasing winding tensions, are only of limited practicability in ultra-thin films, since an increase in the winding tension can result in irreversible film defects, such as stretching.
It is known from U.S. Pat. No. 3,980,611 that a combination of small, medium-size and large particles can improve film handling, depending on the film thickness. U.S. Pat. No. 3,980,611 achieves this by a combination of large (2.5-10 .mu.m) particles with medium-size (1-2.5 .mu.m) and small (&lt;1 .mu.m) particles, where the following relationship must be satisfied:
______________________________________ C.sub.1 = K.sub.large /T.sup.0.6 C.sub.2 = K.sub.medium /T.sup.0.6 C.sub.3 = K.sub.small /T.sup.0.6 K = K.sub.large + K.sub.medium + K.sub.small .ltoreq. 2510 K.sub.medium /K .apprxeq. 0.3 T = film thickness (here in the range from 0.1 to 3 .mu.m) K.sub.large,medium,small = empirical constants K.sub.large 97-500 K.sub.medium &lt;200 K.sub.small 194-2000 C.sub.1,2,3 = concentration in parts per million ______________________________________
These ultra-thin films have the disadvantage of a relatively high content of large particles having a particle size from 2.5 to 10 .mu.m. For a film thickness of 3 .mu.m to 1 .mu.m or less, these large particles can become nominal breaking points during the production process. In addition, they represent weak points with regard to electrical insulation. A further disadvantage is regarded as being the fact that the relatively high roughness or the high surface elevations caused by the large particles, causes a low capacitance per unit volume in the capacitor.
It is known from EP-A-0 423 402 that films having a thickness in the range from 0.1 to 4 .mu.m can be produced by the addition of inert, secondary-agglomerated, inorganic particles having a particle diameter from 0.05 to 5 .mu.m and a primary, spherical particle (i.e. one having an aspect ratio of from 1.0 to 1.2) having a particle diameter of from 0.05 to 4 .mu.m. The secondary-agglomerated particles are, in contrast to primary particles, smaller particles which group together in the polymer and emulate the action of a larger particle. For secondary-agglomerated particles, the mean particle diameter data is related to the size of the agglomerates and not to the size of the smaller particles. The term inert is taken to mean that the particles do not react with the polymer raw material under the process and processing conditions. Spherical means that the particles come very close to the idealized spherical shape. A measure thereof is the aspect ratio, which is the quotient of the largest and smallest diameters, which is 1 in the case of a perfect sphere.
In the case of either a combination of inert, secondary-agglomerated particles with larger, inert, inorganic or organic particles, or in the case of spherical particles, relatively large particles are employed relative to the desired film thickness of .ltoreq.2 .mu.m. Although the addition of the particles simplifies winding of the films or makes winding possible at all, the large particles in these combinations can again represent nominal breaking points during film production and weak points with respect to electrical insulation.
These spherical particles used in the preparation of raw materials and the production of films is likewise described in EP-A-0 236 948, EP-A-0 262 430 and EP-A-0 257 611. However, these applications do not teach the topography required of an ultra-thin film to allow processing without the stated problems of folding and stretching.
Our own experiments have shown that a reduction in the film thickness for the same raw material formulation, i.e., the same chemical composition, makes winding of films more difficult. This is due to the fact that the air trapped between the film layers, which must escape from the winding in the shortest possible time, causes increased irreversible stretching with decreasing film thickness. This irreversible stretching is due to bubble formation caused by trapped air. A measure of the tendency of the film to form such winding defects is the surface gas-flow resistance. This is defined as the time required by air to compensate for a pressure difference between a film and a glass plate. This parameter allows determination of the speed, and therefore the time, with which the air trapped between the individual film layers can escape from the winding. Our own investigations have shown that, for a constant chemical composition of the film, the surface gas-flow resistance depends primarily on the film thickness and film roughness. For example, the surface gas-flow time of a polyethylene terephthalate (PET) raw material containing 1000 ppm of an inert, secondary-agglomerated, inorganic particle having a particle diameter of 0.005 to 4 .mu.m and 1000 ppm of a further inert, inorganic particle having a particle diameter of 0.05 to 5 .mu.m, is shown in Table 1 for film thicknesses of 1.8 to 10 .mu.m.
TABLE 1 ______________________________________ Film Gas-flow time thick- calcu- Roughness ness measured lated.sup.(1) R.sub.a R.sub.z Wind- (.mu.m) (sec) (sec) (nm) (nm) ing.sup.(2) ______________________________________ 10 70 .+-. 20 64 -- -- + 9 -- 81 -- -- + 8 -- 104 -- -- + 7 130 .+-. 50 140 -- -- + 6 180 .+-. 50 196 51 .+-. 8 569 .+-. 130 + 5 260 .+-. 50 292 46 .+-. 8 515 .+-. 130 + 4 -- 475 -- -- + 3 1,180 .+-. 150 889 45 .+-. 8 484 .+-. 130 + 2.5 1,420 .+-. 150 1,324 39 .+-. 8 441 .+-. 130 + 2 2,220 .+-. 200 2,156 36 .+-. 8 403 .+-. 130 - 1.8 2,250 .+-. 200 2,713 34 .+-. 8 384 .+-. 130 - ______________________________________ .sup.(1) Calculated Gas-flow time t = a .multidot. d.sup.b [sec], where a = 9792 [sec/.mu.m] b = -2.18335 d = film thickness [.mu.m] Parameters (a) and (b) were determined empirically. .sup.(2) Winding (+) = films could be wound without folding and stretching (-) = folds and stretching occurred during winding
It can be seen from Table 1 that the surface gas-flow times increase, i.e. the risk of air inclusions and thus of irreversible stretching increases, with decreasing film thickness.
The dependence of the surface gas-flow time on the thickness of ultra-thin capacitor films is shown in illustrative terms in Table 2 for some films having thicknesses of 1.2 to 3 .mu.m (these had different empirical constants than the thicker films in Table 1).
TABLE 2 ______________________________________ Film Gas-flow time thick- calcu- Roughness ness measured lated.sup.(1) R.sub.a R.sub.z Wind- (.mu.m) (sec) (sec) (nm) (nm) ing.sup.(2) ______________________________________ 3 900 .+-. 150 898 43 .+-. 8 468 .+-. 130 + 2.5 1,050 .+-. 150 1,081 42 .+-. 8 436 .+-. 130 + 2 1,250 .+-. 100 1,355 36 .+-. 8 383 .+-. 130 + 1.5 2,000 .+-. 200 1,815 35 .+-. 8 378 .+-. 130 + 1.2 2,300 .+-. 200 2,276 34 .+-. 8 310 .+-. 130 + ______________________________________ .sup.(1) Gas-flow time t = a .multidot. d.sup.b [sec], where a = 2,739 [sec/.mu.m] b = -1.01479 d = film thickness [.mu.m] .sup.(2) Winding (+) = films could be wound without folding and stretching (-) = folds and stretching occurred during winding
Given the above-mentioned relationship between film thickness and gas-flow time, it is not surprising that currently commercially available ultra-thin capacitor films such as Lumirror.sup..circle-solid. C60, Mylar.sup..circle-solid. C or Hostaphan.sup..circle-solid. have comparable surface gas-flow times for the same thicknesses (see Table 3a).
TABLE 3a ______________________________________ Film Gas-flow thick- time ness measured R.sub.a R.sub.z Film type (.mu.m) (sec) (nm) (nm) ______________________________________ Lumirror.sup..circle-solid. 2.0 1,200 .+-. 150 41 .+-. 8 390 .+-. 130 C60 Mylar.sup..circle-solid. C. 2.0 900 .+-. 150 30 .+-. 8 370 .+-. 130 Hostaphan.sup..circle-solid. 2.0 1,250 .+-. 150 36 .+-. 8 380 .+-. 130 ______________________________________
The Hostaphan and Lumirror films which have very similar gas flow times also have very similar roughness values (see Table 3a) and peak height distribution value (see Table 3b). When compared with these films, the roughness value for the Mylar film is lower.
TABLE 3b ______________________________________ Total Surface elevations having a certain number peak height of peaks 0.05-0.3 .mu.m/ 0.3-0.6 .mu.m/ 0.6-1 .mu.m/ Film type 0.36 mm.sup.2 0.36 mm.sup.2 0.36 mm.sup.2 0.36 mm.sup.2 ______________________________________ Lumirror.sup..circle-solid. 15,201 .+-. 14,636 .+-. 513 .+-. 70 52 .+-. 20 C60 2000 1,970 Mylar.sup..circle-solid. C 7,453 .+-. 6,903 .+-. 448 .+-. 70 102 .+-. 20 2000 1,970 Hostaphan.sup..circle-solid. 11,932 .+-. 11,681 .+-. 204 .+-. 70 47 .+-. 20 2000 1,970 ______________________________________
Nevertheless despite the lower roughness value, the Mylar film is found to have a comparable gas-flow time to Hostaphan and Lumirror. This time is achieved through a higher proportion of high film elevations (0.6-1.0 .mu.m), caused by a correspondingly large particle (see in this respect U.S. Pat. No. 3,980,611), than in Hostaphan and Lumirror. However, large particles have the above-mentioned disadvantages of nominal breaking points and electrical defects.
Although films having a thickness of 2 .mu.m and a mean roughness of 30 nm are known (see Tables 3a and 3b), these films have some high peaks in the surface elevations in the range from 0.6 to 1 .mu.m, which are disadvantageous. These peaks are caused by particles whose particle diameter is in some cases significantly greater than the thickness of the film. As discussed above, such large particles are disadvantageous in films having a thickness of .ltoreq.2 .mu.m since tears occur during film production. Very large particles can also have a disadvantageous effect on the frequency of electrical defects, i.e. they can cause an increased number of dielectric breakdowns. The relatively large number of large film elevations is undoubtedly the reason why the films having a mean roughness of 30 nm also have a satisfactory surface gas-flow resistance and accordingly can be produced and processed further without problems (folds and stretching). However, this is at the expense of a reduced capacitance in the capacitor (larger layer separation in the winding).